1. Field of the Invention
The present invention relates to an image reading apparatus and, particularly, to such an apparatus which can read a fluorescence image with high sensitivity.
2. Description of the Prior Art
A fluorescence detecting system labeling a specimen with a fluorescent substance and reading a fluorescence image, is known. According to this system, it is possible to study a genetic sequence, the expression level of a gene and the metabolism, absorption, excretion path and state of a substance introduced into a test mouse and to effect the separation or identification of protein or the estimation of the molecular weight or properties of protein or the like by the steps of labeling a specimen using the fluorescent substance, irradiating the specimen with radiation, exciting the fluorescent substance contained in the specimen and detecting the released fluorescent light. For example, this system can perform a process including the steps of electrophoresing a plurality of DNA fragments on a gel support after a fluorescent dye was added to a solution containing a plurality of DNA fragments to be electrophoresed or distributing a plurality of DNA fragments on a gel support containing fluorescent dye or dipping a gel support on which a plurality of DNA fragments have been electrophoresed in a solution containing fluorescent dye, thereby labeling the electrophoresed DNA fragments, exciting the fluorescent dye by a stimulating ray to cause it to release a fluorescent light, detecting the released fluorescent light to produce an image and detecting the distribution of the DNA fragments on the gel support. This system also performs a process including the steps of electrophoresing a plurality of DNA fragments on a gel support, denaturing the DNA fragments, transferring at least a part of the denatured DNA fragments onto a transfer support such as nitrocellulose support by the Southern-blotting method, hybridizing a probe prepared by labeling target DNA and DNA or RNA complementary thereto with the denatured DNA fragments, thereby selectively labeling only DNA fragments complementary to the probe DNA or probe RNA, exciting the fluorescent dye by a stimulating ray to cause it to release a fluorescent light, detecting the fluorescent light to produce an image and detecting the distribution of the target DNA fragments on the transfer support. This system can further perform a process including the steps of preparing a DNA probe complementary to DNA containing a target gene labeled by a labeling substance, hybridizing it with DNA on a transfer support, combining an enzyme with complementary DNA labeled by a labeling substance, causing the enzyme to contact a fluorescent substance, transforming the fluorescent substrate to a fluorescent substance having a fluorescent light release property, exciting the thus produced fluorescent substance by a stimulating ray to release fluorescent light, detecting the fluorescent light to produce an image and detecting the distribution of the target DNA fragment on the transfer support. This fluorescent detecting system is advantageous in that a genetic sequence or the like can be easily detected without using a radioactive substance.
The image reading apparatus for reading the fluorescent image carried on the image carrier is constructed so as to scan the image carrier with the stimulating ray to stimulate the fluorescent substance in the carrier and to photoelectrically detect the fluorescent light emitted from the fluorescent substance by a light detector such as a photomultiplier. In order to improve the sensitivity of the detection, a color glass filter is provided in front of the light detector to cut light having the same wavelength as the stimulating ray and transmit light having longer wavelength than that of the stimulating ray,
However, the color glass filter tends to emit flourescent light upon irradiation by the stimulating ray. As it is unavoidable that the stimulating light reflected in the apparatus stimulates the color glass filter, the flourescent light emitted from the color filter is photoelectrically detected by the light detector and degrades the sensitivity in image reading.